This invention relates to modified graphite particles suitable as an active material of batteries, such as nonaqueous secondary batteries, and a battery using the same as an active material.
Nonaqueous secondary batteries using lithium ions have a high energy density and a high energy efficiency and provides a higher voltage than other types of batteries. Therefore, they have been studied chiefly for application as a small to ultra-small power source to small-sized and cordless electronic equipment, such as mobile phones, personal handyphone systems (PHSs) and notebook personal computers. Recently, they have also been expected as large batteries having a capacity exceeding 10 Ah that would be useful as a power source of electric cars or hybrid cars or as a small scale power storage means in houses, stores or small factories.
Nonaqueous secondary batteries heretofore used generally comprise metallic lithium as an anode active material. Recent years have seen development of nonaqueous secondary batteries which comprise an anode made of a porous carbon material as an active material in place of metallic lithium, a cathode made of a transition metal oxide containing or not containing lithium, and a nonaqueous organic electrolytic solution comprising a lithium salt electrolyte in a nonaqueous organic solvent. Nonaqueous secondary batteries of this type are now under study for practicability particularly as the above-mentioned large batteries.
The electrode reaction taking place during charging of the nonaqueous secondary batteries of the above type consists of deintercalation of lithium ions from the cathode active material into the electrolytic solution and intercalation of the lithium ions from the electrolytic solution into the porous carbon material of the anode. The electrode reaction during discharging consists of liberation of the lithium ions from the anode carbon material into the electrolytic solution and intercalation of the lithium ions from the electrolytic solution into the cathode active material.
These batteries possess not only the characteristics essential to the nonaqueous secondary batteries using metallic lithium as an anode active material, i.e., high energy density and high energy efficiency, but the merits of not using metallic lithium. That is, they are of high safety and free of such problems as possible reactions between metallic lithium and the electrolytic solution or so-called dendrite formation and are therefore expected to have an extended cycle life.
The porous carbon materials include coke, sintered resins, carbon fiber, pyrolytic carbon, natural graphite, artificial graphite, and mesophase spheres. Particularly preferred of them is natural or artificial lumpy or flaky graphite which is capable of intercalating a large quantity of lithium ions to provide a large theoretical charge and discharge capacity per unit weight.
However, graphite particles are amorphous and contain large particles with great scatter in particle size, which gives rise to the following problems if they are used as such.
(1) The packing density in the anode is low.
(2) Reaction sites in the charge/discharge reactions are concentrated on the angular edges of graphite particles, resulting in non-uniform reactions.
(3) The large specific surface area of graphite particles increases the loss of lithium ions which is attributed to formation of film of a lithium compound on the particle surface in the first charge and discharge cycle.
As a result, the charge/discharge capacity of the anode as a whole decreases, failing to approach to the theoretical capacity of graphite.
The film of a lithium compound as referred to above is considered to consist mainly of lithium fluoride, which is formed by electrodeposition of a lithium salt present in the organic electrolytic solution on the surface of graphite particles, and to contain lithium hydroxide, lithium carbonate, etc., which are formed by reaction between the lithium salt and surface water of the graphite particles or air.
In addition, the following problems arise from use of the amorphous graphite particles.
(4) Due to the above-described reduced charge/discharge capacity, particularly a reduced discharge capacity, lithium ions are gradually accumulated in the graphite particles with repetition of charges and discharges to increase the loss of lithium ions.
(5) Expansion and contraction of graphite particles on charging and discharging vary among particles, and the individual particles show anisotropy in expansion and contraction. Larger particles have larger expansion and contraction. Repetition of expansion and contraction of the particles in the anode tends to cause cracks, which interferes with electrical conduction.
As a result, the batteries have a reduced cycle life.
In order to solve these problems, modification of graphite particles to make them circular or spherical and classification to regulate the particle size within a narrow range have been studied.
For example, JP-A-11-45715 discloses modified graphite particles having a disc or a tablet form, which are obtained by spouting a liquid or gaseous dispersion of amorphous lumpy or flaky amorphous graphite particles from nozzles under pressure in a helical jet to make the particles collide against each other thereby to crush and round the particles and then regulating the particle size by classification. (The term xe2x80x9cJP-Axe2x80x9d as used herein means an xe2x80x9cunexamined published Japanese patent applicationxe2x80x9d)
JP-A-11-263612 discloses modified graphite particles having a prescribed degree of circularity, the cross-section of which looks like a cabbage, which are obtained by a batch process comprising making flaky graphite particles to collide against each other by jet streams in a collision zone thereby to crush and round the particles while removing undersize particles by means of a cyclone type classifier provided above the collision zone.
According to these teachings, the modified graphite particles are expected to provide batteries with an improved charge/discharge capacity and an extended cycle life as compared with non-modified ones. Further, considered theoretically, repetition of the proposed modifying treatment or prolongation of the treating time ought to provide modified graphite particles having a higher degree of circularity and a smaller particle size with a reduced size variation, which are supposed to bring further improvements on both charge/discharge capacity and cycle life.
More specifically, it seems that an improvement in charge/discharge capacity can result for the following reasons.
(i) The packing density of graphite particles in the anode will increase with an increase in degree of circularity, a reduction in particle size, and a reduction in particle size variation.
(ii) As the degree of circularity increases, there will be less angular edges where the reaction sites of charge and discharge reactions may be concentrated, whereby the reactions in the anode will become more uniform.
(iii) The higher the degree of circularity, the smaller the specific surface area as calculated geometrically. As a result, the loss of lithium ions due to film formation on the particle surface will be reduced.
Regarding a cycle life, it is considered that an improvement can result for the following reasons.
(iv) The charge/discharge capacity, particularly the discharge capacity being thus improved, accumulation of lithium ions in graphite particles in repetition of charge and discharge cycles will be suppressed.
(v) With an increase in degree of circularity, a reduction in particle size, and a reduction in particle size variation, the difference among graphite particles in expansion and contraction on charging and discharging will be leveled; the isotropy of the individual particles in expansion and contraction will be improved; and the amount of expansion and contraction of the individual particles will be decreased. As a result, the anode hardly develops cracks due to repeated expansion and contraction.
According to the present inventorsxe2x80x2 investigation, when the modifying treatment as taught in each of the above-described related techniques was continued until the graphite particles are reduced in average particle size to about 18 xcexcm or smaller, further prolongation of the cycle life was obtained. With regard to the charge/discharge capacity, however, it turned out that the capacity had reached its peak before the average particle size was reduced to the above-described level and further continuation of the treatment brought about no increase of the capacity.
An object of the present invention is to provide novel modified graphite particles which, when used as an anode active material, provides batteries having not only an excellent cycle life but the charge/discharge capacity further raised toward the theoretical capacity than as attained by those using conventionally proposed modified graphite particles.
Another object of the invention is to provide a novel battery which exhibits improved charge and discharge characteristics by using the above-described modified graphite particles.
First of all, the present inventors have investigated into configurational changes of graphite particles made by the conventional modifying treatments and found the following facts.
According as the conventional graphite particle modifying treatment proceeds to reduce the particle size, there is a tendency that a rate of size reduction ascribed to breakage of the tabular particles at their periphery becomes smaller than that ascribed to folding of the tabular particles. It follows that a tabular particle is folded several times into a cabbage-like particle as described in JP-A-11-263612.
However, folding a particle to reduce its size makes substantially no change in specific surface area. Therefore, the rate of reduction in specific surface area of graphite particles, which should accompany an increase of degree of circularity and has a great influence on the charge/discharge capacity of a battery, gradually slows down with the progress of the modifying treatment. In other words, the specific surface area of the resulting folded particle is far larger than estimated by geometric calculation based on what should have been achieved when a particle reduces its size by crush and breakage to increase its degree of circularity. As a result, a size reduction achieved by continuing the treatment is not accompanied with an expected increase in charge/discharge capacity.
It is the broken faces of a tabular graphite particle across the main planar direction of the particle that are ready to intercalate and deintercalate lithium ions. The broken face is, on the molecular level, the face where spaces between layers of 6-membered carbon rings, which are characteristic of graphite molecules, crop out to readily accept or release lithium ions. In this regard, particles folded into smaller particles do not expose much of their broken faces so that their capability of intercalating and deintercalating lithium ions is not exerted to the full. This seems to be one of the causes of the failure to sufficiently increase the charge/discharge capacity.
As a result of further investigation, the inventors have ascertained that graphite particles modified into almost circular and tabular particles by breaking or slightly folding their periphery have (1) more of broken faces ready to intercalate and deintercalate lithium ions exposed than the cabbage-like modified graphite particles and (2) a smaller specific surface area as estimated by geometric calculation in good agreement with the reduction in particle size and the increase in degree of circularity. The charge/discharge capacity of batteries using such particles as an anode active material further raises than as attained by conventional batteries and approaches the theoretical capacity more closely.
Based on this finding, the inventors have researched the range of an average particle size Ap (xcexcm) and the range of a BET specific surface area Sc (m2/g) of graphite particles that will secure further extension of the cycle life and further increase of the charge/discharge capacity than heretofore achieved. They have discovered as a result that specifying not only these ranges per se but their ratio, Ss/Ap, is effective for obtaining a further improvement on charge/discharge capacity of a battery while controlling an increase in total surface area of the particles packed up to make an anode. Further study about these ranges and their ratio have led the inventors to complete the present invention.
The present invention provides modified graphite particles having an almost circular and tabular shape, an average particle size Ap of 12 to 18 xcexcm, a BET specific surface area Ss of 4 to 6 m2/g, and a BET specific surface area to average particle size ratio, Ss/Ap, of 0.35 or less which are obtained by treating tabular particles of natural or artificial lumpy or flaky graphite to break or slightly fold the periphery of the tabular particles to increase the degree of circularity.
The present invention also provides a battery having the above-mentioned modified graphite particles as an anode active material. The battery of the invention has an improved cycle life and an improved charge/discharge capacity and, in particular, excellent discharge characteristics owing to the excellent characteristics of the modified graphite particles.